Surface treated lithographic plates and production thereof



United States Patent 3,247,791 SURFACE TREATED HTHOGRAPHEC PLATES ANDPRODUCTION THEREUF Robert F. Leonard, East Rockaway, N.Y., assignor toLitho Chemical and Supply (In. line, Lynhrook, N.Y., a corporation ofNew York g No Drawing. Original application May 6, 1960, Ser. No.27,240. Divided and this application May 4, 1962,

Ser. No. 192,329 (Cl. 1i)1149.2)

dustry for printing plates have recognized disadvantages.

One of the serious disadvantages is that they become oxidized and hencethe chemical properties of their surfaces continually undergo change.They thus present a storage problem due to oxidation and changeundernormal storage conditions. Another disadvantage of known plates isthat they require counteretching prior to the application of thelight-sensitive coating. Where albumin or equivalent sensitizer is lefton non-image areas, scrumming occurs as these areas tend to pick up inkduring use of the plates and such in most undesirable as defectiveprints results despite the use of desensitizers for removing undevelopedsensitizer from the non-image areas. Such plates have a distincttendency to accumulate sensitizer scrum or to tone up during use.Conventional plates are also subject to corrosion during prolongedperiods of storage. Prior plates, other than silicated plates, whichhave been presensitized are apt to deteriorate during storage so thatthey are not fit for use merely upon exposure to light through anegative or stencil and then washing away the unexposed light-sensitivematerial.

Such plates, in particular, especially if they have undergone change ordeterioration do not receive and hold well the light-sensitive diazoresin or other light-sensitive coating or maintain substantiallyconstant permanency of condition. Prior plates are not characterized bylong press life or freedom from halation. These and other disadvantagesare overcome by the present invention.

It has now been found that by treating metal sheets withphospho-molybdate solutions and subsequently subjecting the thus-treatedsheets to scaling operations, there results a conversion coating on thesurfaces of the metal sheets and the so-obtained phospho-molybdatetreated metal sheets combine the desirable physical properties of theoriginal sheets with the below-stated and other advantages of aphospho-molybdate surface. The sheets so produced are ideally suited forprinting plates.

A phospho-molybdate treated metal sheet responding to this invention ascompared with conventional aluminum and zinc sheets now used in thelithographic industry for printing plates has a number of importantadvantages and considerably greater versatility.

A phospho-molybdate treated plate does not oxidize under normal storageconditions. The chemical properies of the surfaces are thereforesubstantially constant and the storage problem is eliminated. Incontrast, conventional zinc and aluminum sheets do oxidize and hencecontinually change with respect to the chemical properties of theirsurfaces and they thus present a storage problem.

A phospho-molybdate treated sheet has a permanent, durable, hydrophilicsurface. A printing plate which does not have a hydrophilic surface inthe non-printing 3,247,791 Patented Apr. 26, 1966 areas has a tendencyto scum because the non-image areas of the printing plate becomesink-respective. The phospho-molybdate treated plate with a hydrophilicimage greatly reduces or eliminates scurnming.

A phospho-molybdate treated sheet does not require counteretching priorto the application of the light-sensitive coating to the sheet whereascounteretching is standard procedure in the processing of zinc andaluminum plates of conventional character. Counteretching involvesscrubbing the plates with an acidic solution to remove loose soil andoxide neither of which is present on the surface of a phospho-molybdatetreated plate or sheet.

The phospho-molybdate treated plate has the further advantages over aconventional lithographic plate in that all traces of casein, albumin,etc. can be positively removed from the plate in the first instancewithout dependence upon any separate chemical desensitizing procedure.

The phospho-molybdate treated plate is characterized by being utilizablefor all three types of lithographic processes, namely the Deep EtchProcess, the Surface Process and the Wipe-On Process. In the Deep EtchProcess, 21 bichromated-gum arabic solution is dried onto the platesurface, exposed through a photographic positive to a light source,developed with an acidic aqueous salt solution, thereby washing awayunexposed sensitizer,

etched with an acidic ferric chloride solution, optionally plated withelectroless copper, lacquered and inked. The light hardened stencil isthen washed off the plate. The areas of the plate which contained thestencil are now the non-printing areas while the area which have beendeveloped, etched, copperized, lacquered and inked are the printingareas. I In the Surface Process, a bieromated casein solution is driedonto the plate surface, exposed through a photographic negative to alight source, coated with a layer of lacquer and then ink, and developedwith a slightly ammoniacal solution. The areas of the plate whichcontained the unexposed, developed (washed away) sensitizer become thenon-printing areas while the lightexposed, lacquered and inked areas arethe printing areas. In the Wipe-On Process, a diazo resin solution or adiazo resin combined with a colloid solution is hand coated onto theplate surface, the plate is next exposed through a photographicnegative, coated with a layer of ink and developed with an acidic gumarabic solution or the exposed plate may be lacquered and developed inone operation with the use of a lacquer emulsion developer. Theunexposed areas of the plate which were developed and washed away becomethe non-printing areas of the plate while the exposed areas of the platewhich were lacquered or inked become the printing areas of the plate.

While conventional zinc and aluminum plates can be used in the Deep Etchand Surface Processes, they have a tendency to scum and must first becounteretched.

Light-sensitive diazo resins and similar light-sensitive materials arenotably sensitive to metals and thus a conventional zinc or aluminumplate cannot be used for wipeon platemaking, whereas a phospho-molybdatetreated plate is ideally suited for the Wipe-On Process,

A silicated plate can be used for wipe-on platemaking, but cannot bepenetrated with the etches commonly used in deep etch platemakingwhereas a phospho-molybdate treated plate is suitable for deep etchplatemaking since it can be etched with conventional deep etch etches.

A lithographic plant could, if so desired, use and store a single typeof lithographic printing plate for all three lithographic platemakingprocesses referred to above by using a phospho-molybdate treated zinc oraluminum plate according to the present invention which, however, alsoincludes the use of phospho-molybdate treated copper sheets or plates.

The phospho-molybdate treated plate can be presensitized withlight-sensitive diazo resins and other like lightsensitive organicmaterials and can then be stored for periods up to several monthswithout loss of photographic sensitivity.

Phospho-molybdate treated plates according to this invention have beenproduced with two different types of surface roughness, namely smoothsurface" plates produced by chemical etching and grained plates producedby metchanical surface treatment. On a theoretical basis, the smootherthe surface of a lithographic plate the greater is the resolution ofhalftone dots; on the other hand, the greater the surface roughness ofthe plate the greater is the capacity of the plate to carry ink andwater. There are, consequently, two types of plates in general use, thesmooth surface plate exemplified by the chemically etched plate and thegrained plate exemplified by the mechanically surfaced plate.

This invention is applicable both to smooth surface" plates and tograined plates as illustrated by the following non-limitative examples:

EXAMPLE I Production of a phospho-molybdate treated sheet with achemically etched surface A x x 0.006 sheet of Alcoa 28 aluminum wasimmersed for 2 minutes at 140 F. in a combination cleaner and etchprepared by mixing the following ingredients in the order listed:

Water ml 3785 Sodium phosphate tribasic grams 28.35 Sodium hydroxide do141.75 Wetting agent (Tergitol non-ionic NPX) do .38

(Alkyl phenyl ether of polyethylene glycol) sold by Union Carbide Thesheet was rinsed thoroughly with deionized water at 68 F., then immersedin a desumitting bath prepared as follows:

rnl. Nitric acid (70%) 1500 Hydrofiuoric acid (5255%) 500 at roomtemperature for 30 seconds. The slightly grained aluminum was thoroughlyrinsed with deionized water at 68 F. before being immersed in thephospho-molybdate bath at 165 F. for 2 minutes. The bath was prepared bymixing the fol owing in the order listed:

Water rnl 7000 Molybdic acid grams 175.2 Sodium phosphate tribasic do34.9

After rinsing thoroughly with deionized water at 68 F., the treatedaluminum was immersed in a sealing bath for 2 minutes at 210 F. The bathis composed of the following:

Water ml 9900 Sodium acetate grarns 100 The sheet was thoroughly rinsedwith deionized water at 68 F. and finally forced air dried.

EXAMPLE 11 Production of a phospho-molybdate treated sheet with amechanical grain A x 22" x 0.12" sheet of Alcoa 25 aluminum was cleanedand degreased by immersion at 68 F. for 15 minutes in the following:

Water liters 4.8 Ethylene glycol rnonoethyl ether do 3.2 Sodiumcarbonate grams 86 Sodium phosphate tribasic do 28.4

The sheet was rinsed with water at 65 F. for 2 minutes before beinggrained. The graining consisted of passing the sheet in a horizontalplane under a set of revolving brass brushes which are located on theperimeter of a rotating circular support. During this operation, theplate is flushed with a slurry of water and pumice powder. The aluminumsheet was rinsed thoroughly with water at 65 F. and then immersed in theabove water, ethylene glycol monoethyl ether, sodium carbonate, sodiumphosphate tribasic cleaner for 5 minutes at 68 F. The plate was thenrinsed thoroughly with water at 65 F. and then immersed in thephospho-molybdate solution at 160 F. for 2 minutes. The bath consistedof the following:

Water "gallons" 30 Molybdic acid grams 2480 Sodiurnphosphate tribasic do566 The sheet was rinsed for 2 minutes with tap water-at 65 F. beforebeing immersed in the sealing bath at 210 F. for 2 minutes. Sealing bathconsisted of:

Water gallons 30 Sodium acetate grarns 1225 A The sheet was rinsed withwater at 65 F. for 2 minutes,

then forced air dried. Finally, the sheet was passed at a distance ofsix inches over a series of lighted gas burners for 4-0 seconds tocompletely seal the phospho-molybdate surface.

The molybdic acid referred to in Examples I and II is molybdic acidwhich is a commercially known product composed of ammonium paramolybdatewith added molybdic oxide. The molybdic oxide amounts to 85% of theweight of the product which produces molybdic oxide when the product isdissolved in water. The molybdic oxide combines with phosphate ionspresent in the treating bath to form phosphomolybdate ions in solution.The sodium and ammonium ions present in the phosphomolybdate solution,resulting from the combining of the molybdic acid 85% and the tribasicsodium phosphate, remain as free ions as long as they are in solution.The phosphomolybdate ions can, however, be produced in other wayswithout adversely affecting the working characteristics of the treatmentbath. Either of the following baths A or B can be used in place of thoseof Examples I and II, viz.:

Water ml 1000 Sodium phosphomolybdate (Na PO -l2MoO grarns 20 Sodiumhydroxide do 5 Bath A was used on a chemically grained aluminum plate atto F. for 2 minutes. The remainder of the treatment was the same asExample I.

Water ml 1000 Ammonium molybdate [(NH4)6MO7O24'4H2O] "grams" 50Phosphoric acid (85%) ml 1.25 Sulfuric acid (98%) ml 1.00

Bath B was used on a chemically grained aluminum plate at F. for 2minutes. The remainder of the treatment was the same as Example I.

It has been found that, in regard to treating metallic sheets, the bestphosphomolybdate solutions are those which contain 12 to 16 moles ofmolybdic oxide to 1 mole of phosphate. It appears, therefore, thateither phospho- 12-molybdate ions or phospho-l8-molybdate ions, or acombination of both, are formed in solution and ultimate- 1y react withthe aluminum, zinc or copper to form the corresponding metallicphosphomolybdate.

It has further been found that the above reaction is best promoted,Without attacking the metal sheet, at a pH in the range of 4- to 6.

weight) aqueous solution of sodium acetate.

It is further to be understood that the concentration of solids in thephosphomolybdate bath of Examples I and II can be varied materially aslong as the molybdic oxidephosphate ion molar ratio is maintainedconstant. The bath can thus range in its components as follows:

Water rnl 1000 Molybdic acid, 85% "grams" 9.1 to 37.7 Sodium phosphatetribasic do 1.8 to 7.5

If the molybdic acid 85% and sodium phosphate tribasic concentrationsexceed 37.7 and 7.5 grams, respectively, the phosphomolybdate bath willsoon form a precipitate and become unusable; if, on the other hand, themolybdic acid 85% and sodium phosphate tribasic concentrations are below9.1 and 1.8 grams, respectively, the metal sheet will require longerthan 2 minutes at 155 to 180 F. and

hence the operation becomes uneconomical.

The optimum temperature range for use of the phosphornolybdate solutionsis between 155 F. and 180 F. At a temperature below 155 F. the reactionis slowed down appreciably and the process becomes uneconomical and whenthe temperature is above 180 F. the bath gradually forms a precipitateand has only a limited storage life. a

The following phosphates have been successfully used in varyingproportions to supply the required phosphate ions in the treatment bath:

Phosphoric acid (85%) Ammonium phosphate, monobasic Sodium phosphate,dibasic Sodium phosphate, tribasic No phosphate ion source has beenfound which did not operate satisfactorily as long as the pH of theresulting phosphomolybdate solution was adjusted to pH 4 to 6.

Molybdic acid 85 and ammonium molybdate are suitable sources of molybdicoxide and produce satisfactory results when combined with the properamount of phosphate ion as long as the pH is in the range of 4 to 6.They are equally as good as the sodium phosphomolybdate mentioned above.

The sealing bath described above consists of a 1% (by Tests showed noadvantage in using a sealing bath of concentration and hence a 1%solution is preferred. The sealing bath, is, however, not limited to a1% solution of sodium acetate as any of the following chemicals can alsobe used in a 1% by weight concentration in water at 200 to 212 F. toform a satisfactory sealing bath: dibasic sodium phosphate, potassiumacetate, magnesium acetate, barium acetate, calcium acetate, dibasicammonium carbonate, barium carbonate, calcium carbonate, lithiumcarbonate, magnesium carbonate, zinc carbonate, ammonium hydroxide andurea.

The following chemicals when used in a 1% concentration at 200 to 212 F.did not produce a satisfactory sealing bath:

Ammonium phosphate, monobasic Sodium phosphate, monobasic Sodium.phosphate, tribasic Potassium phosphate, monobasic Magnesium phosphate,dibasic Ammonium acetate Zinc acetate Sodium carbonate, dibasicPotassium carbonate, dibasic Potassium oxalate Ammonium oxalate Sodiumcitrate Potassium citrate Ammonium citrate Calcium citrate Magnesiumcitrate Sodium nitrate Potassium nitrate Ammonium nitrate Calciumnitrate Magnesium nitrate Zinc nitrate Aluminum nitrate Barium nitrateSodium chloride Potassium chloride Ammonium chloride Lithium chlorideCalcium chloride Zinc chloride Barium chloride Sodium sulfate, dibasicPotassium sulfate, dibasic Calcium sulfate Zinc sulfate Magnesiumsulfate From phospho-molybdate treated aluminum plates preparedaccording to Example I, there have been produced nylon-diazopresensitized plates in which the nylon-diazo sensitizer corresponds tothe sensitizer described in US. Patent No. 2,826,501. The nylon-diazosensitizer in conjunction with the phospho-molybdate treated aluminumplate produces a markedly superior plate as compared to the use of thesame nylon-diazo sensitizer in conjunction with a silicated aluminumplate. The plate produced by the nylon-diazo sensitizer on the silicatedsurface is photographically too fast for lithographic purposes whereasthe same sensitizer on a phospho-molybdate surface is ideal forlithographic purposes.

Phospho-molybdate treated aluminum plates prepared according to ExampleII have been very satisfactorily used to produce deep etch plates,surface plates and wipeon diazo plates.

As pointed out above, plates or sheets of zinc, aluminum and copper canall be beneficially phospho-molybdate treated and all are suitable forlithographic printing plates. i

The presensitized plates referred to above are prepared by making achemically grained phosphomolybdate treated plate as already described,applying thereto the nylon-diazo resin solution of US. Patent No.2,826,501 by wiping the same on by hand or by means of a roller and thenair drying the coating thus produced. The pre-' sensitized plate is thenprocessed by exposing the coating to a light source through a stencil ornegative, developing or washing away the unexposed areas with thedeveloper of US. Patent No. 2,826,501, applying a lacquer like that ofUS. Patent Nos. 2,754,279 or 2,865,873, and rinsing with water.

The nylon-diazo resin solution of Patent No. 2,826,501 is composed ofthe following constituents in approximately the following amounts, byweight:

0.11% of the water soluble condensation product of pdiazo diphenylamineand formaldehyde,

0.94% of soluble superpolyamide nylon resin,

0.01% of a non-ionic surface active agent 17.15% of water,

76.35% of denatured ethyl alcohol, and

5.44% of furfuryl alcohol.

The developer of Patent No. 2,826,501 is composed of the followingconstituents:

Citric acid grams 1.2 N,N-dimethylformamide milliliters 124.5 Furfurylalcohol do 55.2 Methanol do 375.0

In a typical application of the deep-etch process, a mechanicallygrained plate produced as in Example II is centrifugally coated with abicromated gum arabic solution according to the recommendations of theLithographic Technical Foundation publication No. 806. Such solutioncontains water, gum arabic, ammonium bichromate, a wetting agent(surfactant), a blue dye and ammonium hydroxide. The basic coatingsolution contains 2840 ml. of 14 Baum gum arabic solution, 950 ml. of-

ammonium bichromate stock solution (758 grams of photo grade ammoniumbichromate in enough water to make 1 gallon of 14.2 Baum at 77 F.), and140 ml. of ammonium hydroxide (28% NH to which the other ingredients areadded. The coating solution has a pH value of 8.8 to 9.0 and testsbetween 14.0 Baum at 77 F. Whirling is continued until the coating driesand the coated plate is then exposed to a light source through asuitable positive. The unexposed areas are developed or washed away withan aqueous acidic salt solution of calcium chloride in water to whichlactic acid has been added according to the recommendations ofLithographic Technical Foundation publication No. 806, e. g.:

Zinc chloride (technical) grs 680 Calcium chloride (commercial) do 1360Water -2 ml 1890 Lactic acid (85%) do 340 The bared metal is etched inacidic ferric chloride solution according to the recommendations of theabove publication, e.g.:

Calcium chloride solution (40-41 B) ml 2630 Zinc chloride (technical)grs 1000 Iron perchloride solution (505l B) ml 750 Hydrochloric acid(3738.5%) ml 37 Cupric chloride grs 70 The plate is next washed withanhydrous alcohol to remove salts and water and a lacquer film appliedby hand to the thus treated plate and allowed to dry, following which adeveloping ink is applied by hand and allowed to dry. The plate is nowsoaked in warm water which penetrates to the exposed coating leaving theplate with inked image areas. The non-printing areas are hydrophilic dueto the previous treatment of the plate.

In a typical application of the wipe-on process, using diazo resinsolution, a mechanically grained plate produccd as in Example II iswiped on with a pool of diazo resin solution and smoothed down with asoft-non-abrasive applicator and allowed to air dry. The diazo resinsolution is a 1 to 5%, by weight, aqueous solution of the condensationproduct of p-diazo diphenylamine and formaldehyde. The plate is nextexposed to actinic light rays through a photographic negative and isfinished in either of the following ways:

A. A developing ink of known composition is applied in the same manneras the diazo resin solution and developed with a developer such as onecomposed of water, gum arabic and phosphoric acid. The developerpenetrates through the ink and removes, by dissolving, the unexposeddiazo sensitizer coating. The image areas are exposedwater-insolubilized diazo resin covered by greasy ink and thenon-printing areas are hydrophilic phosphomolybdated surfaced.

B. A lacquer emulsion the same as or similar to that of United StatesPatent No. 2,865,873 is applied. The image areas are exposedwater-insolubilized diazo resin covered by the lacquer composition andthe background or non-printing areas are hydrophilic phosphomolybdatedtreated surfaces.

The surface process is carried out according to the recommendations ofthe Lithographic Technical Foundation publication No. 807, the contentsof which are hereby made a part hereof. The plates are termed surfaceplates and are plates which have been exposed through negatives and onwhich the exposed coating serves as a base for the ink-receptive image.In general, an aluminum sheet is grained in known manner and cleanedwith a counteretch such as acetic acid in water. A light-sensitivecoating is applied, with or without (usually without) a prior pre-etchwith the same plate etch used for desensitizing. The coating solution isa mixture of a solution of ammonium bichromate and a solution of acolloid such as albumin, casein, gum arabic, gelatin or cellulose gum.The coated plate is exposed to light through a negative to form theimage or printing areas. Light passing through the clear portions of thenegative hardens or tans the coating under these areas. Any suitableavailable commercial lacquer is applied to the exposed surface plate andserves as protection for the image areas. A developing ink is thenapplied to the expoesd plate to place a greasyink-receptive layer on theimage areas and the plate is developed to remove unexposed coating fromthe non-image areas. The usual finishing operations are then carriedout, all as described in the said publication #807.

What is claimed is:

1. A method of preparing a sealed phospho-molybdate treated lithographicplate which comprises reacting a cleaned metal plate selected from thegroup consisting of zinc, copper and aluminum plates in an aqueous bathconsisting essentially of phospho-molybdate ions in solution to form onthe plate surfaces the phospho-molybdate of the plate metal and thenimmersing the phosphomolybdated plate in an aqueous sodium acetatesolution.

2. A method of preparing a phospho-molybdate treated lithographic platewhich comprises immersing a cleaned metal plate selected from the groupconsisting of zinc, copper and aluminum plates in an aqueous bathconsisting essentially of phospho-molybdate ions to form on the platesurfaces the phospho-molybdate of the plate metal and then immersing thephospho-molybdate coated plate thus formed in a 1% sodium acetatesolution at a temperature of 200 to 212 F. followed by drying.

3. A phosphomolybdate treated plate utilizable for deep etch, surfaceand wipe-on lithographic processes, said plate being constituted of ametal selected from the group consisting of zinc, aluminum and copperthe surfaces of which are composed of the corresponding zincphosphomolybdate, aluminum phosphomolybdate and copper phosphomolybdateprovided with a sealing coating of sodium acetate.

References Cited by the Examiner UNITED STATES PATENTS 597,366 1/1898Strecker 1486.15 1,997,550 4/1935 OLeary 1486.15 X 2,008,733 7/1935Tosterud 148-614 2,234,206 3/1941 Thompson 1486.5 2,328,540 9/1943Hochwalt 1486.15 2,333,206 11/1943 Sloan 148-6.15 X 2,403,426 7/ 1946Douty et al. 1486.15 2,502,441 4/1950 Dodd et al 148--6.15 2,557,5096/1951 Miller 148-6.15 X 3,030,210 4/1962 Chebiniak 1486.15 3,060,06610/1962 Ross et al 148-615 3,071,494 1/1963 Humphreys 1486.1 X

WILLIAM D. MARTIN, Primary Examiner,

3. A PHOSPHOMOLYBDATE TREATED PLATE UTILIZABLE FOR DEEP ETCH, SURFACEAND WIPE-ON LITHOGRAPHIC PROCESSES, SAID PLATE BEING CONSTITUTED OF AMETAL SELECTED FROM THE GROUP CONSISTING OF ZINC, ALUMINUM AND COPPERTHE SURFACES OF WHICH ARE COMPOSED OF THE CORRESPONDING ZINCPHOSPHOMOLYBDATE, ALUMINUM PHOSPHOMOLYBDATE AND COPPER PHOSPHOMOLYBDATEPROVIDED WITH A SEALING COATING OF SODIUM ACETATE.